The present specification generally relates to mobile radio communications with focus on parallel operation of multiple radio access technologies (RAT), in particular Long Term Evolution (LTE) networks in combination with 3rd Generation (3G) Universal Mobile Telecommunications System (UMTS)/High Speed Packet Access (HSPA) and/or with 2nd Generation (2G) Global System for Mobile Communications (GSM)/General Packet Radio Service (GPRS) networks. In particular, the present specification is targeting on self optimizing networks (SON) with focus on the use case “mobility robustness optimization” (MRO) between different RATs. Inter-RAT MRO is discussed in 3rd Generation Partnership Project (3GPP) as part of the LTE Release 11 SON framework.
Inter-RAT mobility describes the cell change of a terminal (e.g. user equipment (UE)) where the source and the target cell belong to different RATs. This cell change can be either caused by UE movement where the UE is leaving the coverage range of a certain RAT or by so-called traffic steering (TS) reasons which might even happen without real user movement.
In the first case the cell change is triggered by the radio condition of the serving and potential target cell. Signal of serving RAT becomes weak and falls below a certain threshold, while signal from measured target cell (of a target RAT) is above a certain threshold.
In the second case, for instance the load situation is decisive rather than the signal quality, which thus plays a secondary role in the second case.
3GPP Release 11 is considering the inter-RAT deployment scenario with limited LTE coverage and, therefore, is focusing on following two inter-RAT mobility failure cases:                Too late inter-RAT handover from LTE to 3G, and        Too early inter-RAT handover from 3G to LTE.        
It is to be noted that there is a further inter-RAT mobility problem without a radio link failure (RLF), namely the inter-RAT ping-pong handovers where the UE is frequently performing handover forth and back between different RATs. The present specification focuses on the latter inter-RAT mobility problem.
In 3GPP, it is also distinguished between necessary and an unnecessary inter-RAT ping-pong handovers.
If a UE is frequently moving between two locations where the one or the other RAT has a coverage hole, inter-RAT ping-pong handovers are considered as being necessary and not as problem.
However, ping-pong handovers might also result from misaligned inter-RAT mobility parameter setting of the two RATs, most likely if radio driven and traffic steering driven handover are triggered in the different RATs. Those inter-RAT ping pongs are considered as being unnecessary.
Reasons for traffic steering driven handover are for instance load balancing among different RATs, energy saving with evacuating one RAT that is intended to be switched off, or RAT preferences for dedicated services.
The detection of inter-RAT ping-pong handovers is based on investigating the UE history information provided with the HANDOVER REQUEST message during handover preparation. The UE history information (information element (IE)) contains a list of the last visited cells with information about the cells themselves (e.g. E-UTRAN cell global identifier (ECGI)) and the time how long the UE stayed in these cells.
Two succeeding handovers are identified as inter-RAT ping pong if the UE history information shows that the last visited cell of the UE was of a different RAT with a rather short stay time (Tstay<Tthres_shortstay) while the cell visited before the last visited one and the current cell belong to the same RAT, where
Tstay_otherRAT is derived from the IE “time UE stayed in cell” as specified in last visited cell information for the cells, and
Tthres_shortstay could be an internal MRO parameter to detect short stays and ping pongs.
The inter-RAT ping pong refers to the RAT layer and is independent of the cell itself. A corresponding MRO counter is incremented if the UE which has been handed over to a different RAT comes back to the previous RAT irrespective if it returns to the same cell which initiated the handover or to a different one.
That is, if for example a LTE cell A is initiating an inter-RAT handover to an UMTS cell X, the following two cases are considered as being inter-RAT ping pong:                Cell_A@LTE to Cell_X@UMTS (to more cells @UMTS) to Cell_A@LTE, and        Cell A@LTE to Cell_X@UMTS (to more cells @UMTS) to Cell_B@LTE.        
Accordingly, in the context of inter-RAT the stay time Tstay_otherRAT is referring to RAT and not to a particular cell. Depending on the duration of Tstay_otherRAT, the UE could be connected to several cells before coming back to the previous RAT. However, ping pong in general means a short stay (i.e. normally in one cell) of the new RAT and subsequent handover back to the initiating RAT.
Irrespective of to which cell the UE returns within LTE, the “guilty” cell which initiated the “short stay” in another RAT is Cell_A and, therefore, responsible for the inter-RAT ping pong. Therefore, in the second case above, an evolved NodeB (eNB) controlling Cell_B when detecting the ping-pong events between Cell_A, Cell_X and Cell_B, will inform the eNB controlling Cell_A using an X2 HANDOVER REPORT message.
Based on the assumptions, if there are any counters implemented to monitor the ping-pongs, those should be collected at the guilty node.
Corresponding to the above mentioned two cases, if for example a UMTS cell X is initiating an inter-RAT handover to an LTE cell A, the following two cases are considered as being inter-RAT ping-pong handover:                Cell_X@UMTS to Cell_A@LTE (to more cells @LTE) to Cell_X@UMTS, and        Cell_X@UMTS to Cell A@LTE (to more cells @LTE) to Cell_Y@UMTS.        
In this case the following difference is to be noted. Namely, the difference on 3G side is that both cells Cell_X and Cell_Y are most likely controlled by the same radio network controller (RNC) and, therefore, no additional signaling is needed. The RNC is responsible for administration of the counters, but those are not specified within 3GPP so far but implementation specific.
Multiple ping pongs and multiple inter-RAT ping pongs are already occurring in existing 2G and 3G networks, but they are not specifically treated or counted in terms performance indicators. Only a considerable increase of the total number of handovers between some cells is remarkable. Accordingly, it is then up to network optimization experts to conclude from such increased number of handovers that “multiple consecutive ping pongs” might have been occurred.
Further, in GSM there are provided features like “Limitation of Intra-cell Handover Repetition” where in an interfered cell consecutive intra-cell handovers are to be avoided, since they cannot improve the quality, or “Prevention of Back-Handover” where handovers back to a cell which has been just left before are to be avoided by labeling the handover with “imperative” or “forced handovers”, such that UEs are not allowed to be handed over back within a defined time frame.
However, the above mentioned features are proactive measures to avoid this sort of unwanted handovers without knowing that a ping pong will occur at all. Further, those measures do not provide any means for (automated) correction or optimization of mobility parameters.
Even though self-optimization of mobility parameters by MRO is going to be established with LTE, a particular treatment of “multiple ping-pong handovers” (i.e. multiple consecutive inter-RAT handovers forth and back) to overcome the issues mentioned above is not considered.
By means of prior art, each single “forth and back handover” would be counted, which may indicate that there is a massive ping pong problem. However, problems implied above and mentioned below (e.g. affecting control plane processing capacity) are not solved.
According to current specifications in 3GPP Rel. 11 SON framework a handling of inter-RAT ping-pong handovers consists of two steps, where in the first step a ping pong is detected and counted. In a second step, the ping pongs are further classified into necessary and unnecessary ones, wherein the unnecessary ones are then counted and reported as inter-RAT ping pongs. That is, these potentially “never ending” forth and back inter-RAT handovers are not considered to be specifically treated.
According to the Published European Patent Application EP 2 465 294 A1, it is suggested to deal with the inter-RAT ping pong problem by an exchange of messages between the nodes to manipulate the setting of the mobility parameters of the counterpart node. However, this document does not deal with the special case of multiple long lasting consecutive inter-RAT ping pongs (ping-pong handovers, handover bounces).
Hence, the problem of multiple consecutive (eventually never ending) inter-RAT ping pongs arises, where the UE is handed over back and forth with a very short connection time. A ping pong as such can be derived from UE history information and therefore the detection process is per se an eNB-internal procedure (or RNC-internal procedure, respectively), and thus implementation specific. The same holds for the detection of multiple ping pongs with many consecutively occurring back and forth handovers between two RATs.
According to the current definition in 3GPP, one single “forth and back handover” is detected as a ping pong and counted as such. In case of multiple consecutive “forth and back handovers” each of them fulfils the criterion for a ping pong and increments the ping pong counter. This leads to the problem that the ping pong counter will dramatically increase and might even risk an overflow of the counter.
A further problem is caused by fact that multiple consecutive ping-pong handovers might be a long lasting procedure and not a single event like a RLF. This long lasting procedure requires and takes worthwhile control plane processing capacity for preparation and execution of all of the ping pong handovers, and implicitly affects the complete cell within both RATs or even the complete nodes controlling these cells. Furthermore, the UE throughput is declining and might even confuse the higher layers (e.g. transmission control protocol (TCP) congestion control). A proper user plane connection can not be established, since the UE is continuously forced for inter-RAT measurements, handover preparations, new synchronization procedures, etc.
Accordingly, long lasting multiple consecutive ping pongs impair both network performance and user satisfaction.
A further problem arises for the (possibly) rare case that the UE returns from 3G alternating to different LTE cells (Cell_A@LTE to UMTS to Cell_B@LTE to UMTS to Cell_A@LTE and so on). Namely, in this case, X2 HANDOVER REPORT will be also sent forth and back between two LTE cells, such that each of the two cells increments the ping pong counter.
Hence, there is a need to provide for detection and inhibition of multiple consecutive inter-RAT ping-pong handovers.